Clearing Your Outlook Forms Cache
You're working in Outlook, minding your own business and someone sends you a meeting request; you check your schedule, you're free at the given date and time so you click on Accept only to see this:
Restarting Outlook doesn't help, nor does rebooting your machine; you've fallen victim to a corrupted forms cache. But don't fret, there's a straightforward, if not entirely intuitive, way of clearing the cache.
Standard disclaimer: YYMV, this worked for me, using Outlook 2003 on XP Professional Service Pack 2, it might work for you, it might not.